function pairs(k, arr) {
    // Write your code here
    let map = arr.reduce((acc, id) => {
        acc.set(id, (acc.get(id) || 0) + 1);
        return acc;
    }, new Map());
    let count = 0;
    for(let [key, val] of map) {
        if(map.has(key-k)) count += val*map.get(key-k);
    }
    return count;
}